Metallized polypropylene film is used in many packaging applications. Its advantages include low permeability to water vapor, brilliant appearance, low density and low cost. However, a major deficiency results from the relatively poor metal-to-polymer bond typically seen in prior art films.
While manufacturing techniques and polymer property requirements are known for the manufacture of metallized polypropylene film, various modifications have been employed to attempt to overcome the adhesion deficiency. These alterations have resulted in polypropylene surface changes Some examples of these modifications include coextrusion with an adhesion promoting polymer, corona discharge or plasma treatment, application of very thin adhesion promoting solutions, dispersions or emulsions just prior to, during or after an orientation process and chemical etching of the film surface with acids, solvents or other reactive agents.
Although currently available biaxially oriented polypropylene films provide some improvement for metal adhesion over that given by a modified homopolymer film, many drawbacks are created. Chiefly, the processes for manufacture of these films involve greater complexity and increased cost. Also, the current processes limit the suitability for use of the film in subsequent converting operations. Specifically, the ability to successfully laminate two or more films by extrusion techniques may be compromised due to the tendency for the metallized layer to crack or craze. Crazing during the extrusion lamination process is a widespread problem which results in poor appearance as well as property deficiencies. It is known that unmodified metallized films do not exhibit this crazing phenomenon during the lamination process, however the metal to film adhesion is deficient.
Metal crazing is believed to be the result of differential stretching between the bulk film layer and the metal adhering layer brought about by heat and stresses introduced by the extrusion lamination process. With most commercial films, there is a trade-off between the ultimate metal bonds attainable and the craze resistance of the film.